1. Field of the Invention
The present invention relates to methods for preparing amides, and more particularly, to a method for preparing amides by catalyzing ketoximes with ionic liquids.
2. Description of the Prior Art
In the recent years, ionic liquids are widely applied in the chemical field. From a practical aspect, ionic liquids are salts with melting points below 100° C., composed of anions and cations, and have special properties including a high polarity, a low vapor pressure, a low melting point, non-inflammability, resistance to strong acids, resistance to high temperatures, high electrical conductivity, an excellent electrochemical property, and a broader liquid temperature range (−96° C. to 400° C.), and thus can be applied in various technologies such as chemical synthesis, catalysis, electrochemistry, analytical chemistry and separation technologies. Because ionic liquids can be used under a normal pressure, they can replace commonly used volatile organic compounds (VOC), so as to reduce contamination of VOC to the environment and avoid exposure of VOC to operators. Further, ionic liquids are recyclable. Hence, ionic liquids are considered as a new green solvent.
Currently, a lot of research works focus on introducing atoms having specific functions or reactivity into the molecular structures of ionic liquids at room temperature to prepare functionalized ionic liquids and give ionic liquids certain special properties, applications or functions, so that they become “task specific ionic liquids”. For example, an ionic liquid having amino at the terminal of a cationic substituent captures carbon dioxide at room temperature and releases it at a higher temperature, and thus can selectively separate carbon dioxide from a mixed gas (J. Am. Chem. Soc., 2002, 124, 927).
Caprolactam is an important raw material in the manufacture of nylon 6 fibers and thin films. Beckman rearrangement of cyclohexanone oxime is an important reaction step in producing caprolactam. Currently, fuming sulfuric acid is used as a catalyst for converting cyclohexanone oxime to caprolactam sulfate during Beckman rearrangement, and then ammonia is used for neutralization, so as to obtain caprolactam. While the conversion rate of cyclohexanone oxime is almost 100% and the selectivity for caprolactam is 99%, a large amount of low-valued ammonium sulfate is generated during the reaction, and concentrated sulfuric acid used for catalysis causes problems such as corrosion to the whole equipment and environmental pollution. In the recent years, researches on new production technologies of caprolactam focus on reducing or avoiding the generation of the by-product, ammonium sulfate. Moreover, compared with the gas phase reaction, liquid-phase rearrangement has advantages including moderate reaction conditions, fewer requirements to the equipments, etc., and is advantageous to the reconstruction of the current equipments. As a result, scholars worldwide have put efforts on developing liquid-phase rearrangement, and attained substantial developments and breakthrough. For example, in U.S. Pat. No. 5,225,547 assigned to Sumitomo Chemical Company Ltd. in Japan, a catalytic system consisting of an alkylating agent and N,N-dimethylformamide (DMF) is used to give the selectivity of caprolactam up to 99.8%. In U.S. Pat. No. 5,571,913 assigned to DSM N.V. in Holland, a cation exchange resin is used as a catalyst to give the selectivity of caprolactam up to 100%.
Accordingly, there is an urgent need of a method for preparing amides by liquid-phase rearrangement of ketoximes without corrosive catalysts.